1. Technical Field
The present invention relates to a measuring apparatus and a measuring method. More particularly, the present invention relates to an apparatus and a method for measuring a characteristic of an object by irradiating the object with an electromagnetic wave and measuring the modulated electromagnetic wave from the object.
2. Related Art
Electromagnetic waves whose frequency substantially falls within a range from 20 GHz to 120 THz or whose wavelength substantially falls within a range of 1.5 cm to 2.5 μm (hereinafter, referred to as “terahertz waves”) appear at the boundary between light and radio waves. The terahertz waves have not been used very much in any technical fields. Applications of terahertz waves, however, have been increasingly considered in many technical fields due to the following achievements. It has become possible to generate short femtosecond terahertz pulses with the use of semiconductor lasers and a technique based on such pulses, which is referred to as terahertz time domain spectroscopy, has been developed.
“Terahertz Time Domain Spectroscopy”, Kiyomi SAKAI, Spectroscopy Studies, Vol. 50, No. 6, Page. 261-273 (2001) discusses the principles of the spectroscopic measurement, imaging, and any other applications using electromagnetic waves in the terahertz band. As mentioned in Non-Patent Document 1, terahertz waves have both a transmittance property similarly to radio waves and a line-of-sight propagation property similarly to light. Thus, the terahertz light can exhibit high resolution while being able to pass through objects and be handled in quasi-optical devices such as lenses and mirrors.
Japanese Patent Application Publication No. 2004-108905 mentioned below discloses a method for detecting the shape, composition and any other characteristics of an object by irradiating the object with terahertz waves having different wavelengths and measuring the transmittance values of the object for these terahertz waves to know the electromagnetic wave absorption by the object. This method can detect any objects that have wavelength dependence of terahertz wave absorption without relying on the surfaces of objects.
J. M. Lamarre et al., “METALLIC MESH PROPERTIES AND DESIGN OF SUBMILLIMETER FILTERS,” Int. Jnl. Of Infrared and Millimeter Waves, Vol. 2, No. 2, 1981, pp. 273-292 and Japanese Patent Application Publication No. 2004-117703 state that a metal plate having periodically arranged spaces (hereinafter referred to as “the space arrangement structure”) excellently transmits electromagnetic waves. This space arrangement structure can be utilized as a retarder by itself, and also used as a support for an object to be measured during measurement using terahertz waves.
When irradiated with terahertz waves, an object to be measured returns a measurement result uniquely determined by the characteristics of the object. For example, when the transmittance of the object is measured, the frequency characteristics of the transmittance of the object have a peak indicating a local maximal value that is uniquely determined by the characteristics of the object.
In light of the above, it is possible to know the characteristics of an object by first measuring the characteristics of a space arrangement structure alone, then measuring the characteristics of a combined structure in which the object is supported by the space arrangement structure, and finally calculating the difference between the measured characteristics. When the frequency characteristics of the transmittance of the object are measured as mentioned above, for example, the peak appears in different bands. In this manner, it is possible to detect whether or not the object is present and may also be possible to identify the composition and any other characteristics of the object depending on the shift of the peak. Since such a change in the measurement results is clearly found, effective and valid measurement is possible even when the quantity of the object is very small.
When measured by using terahertz waves, however, the frequency characteristics of transmittance draw a relatively smooth and continuous curve. Therefore, there are difficulties in detecting a shift when only a small shift is caused by presence of an object. In light of the above, it is desired to provide a measuring method and a measuring apparatus that are capable of detecting the characteristics of an object with more ease with it being possible to maintain the advantages of the measurement using terahertz wave.